Uniform clamp load across a plurality of fasteners is important for providing adequate coupling and reliable joint connections. The desirability of obtaining uniform clamping load can be especially important in certain critical areas of an internal combustion engine such as the main bearing caps for the crankshaft where proper uniform clamping load is essential to attain proper bearing cap functioning and service life. Various bolts or fasteners have been designed to indicate the amount of tension applied, but these are fairly complicated in design and use or require large manufacturing costs. For example, Liber (3,718,065) discloses a tension indicating fastener which includes a photoelastic strip wherein polarized light is directed toward the strip to measure fringe order for determining binding stresses in the head which is linearly related to the bolt tension. This fastener, however, requires a calibration curve to determine bending stress from the fringe order which is not a time efficient system for a user. The tension indication means could also add significantly to the cost of such a bolt.
Torquing methods have also been utilized for application to ordinary fasteners. The traditional Torque Only method merely involves tightening a plurality of fasteners in an accepted pattern to a predetermined amount of torque using a common torque wrench which allows a user to read the amount of torque applied. However, even with an accurate torque wrench reading, the range of clamp loads in bolted joints using the Torque Only method has been found to be +/-25%.
A second method, the Torque Angle Method, has been developed which provides a range at joint connections of only +/-10%. This method requires at least three steps; first, the fastener is tightened to an intermediate torque common to each of the plurality of fasteners, second, a mark is scribed on the clamped element adjacent to the fastener head, such as in alignment with one of the six points on the hexagonal head typically provided. Next, the fastener is rotated a desired angular distance such as by aligning a succeeding point on the hexagonal head with the mark scribed on the clamped element. One of the disadvantages of this method is that in field service applications a user is limited to use of the points on the hexagonal head or must manually scribe 2 lines on the fastener separated by the predetermined angle. This requires time to mark the fastener, results in inaccuracies of the lines themselves, or generally limits a working angle to increments of 60 degrees because it is more expedient and efficient to scribe the clamped element in alignment with one of the points on a hexagonal head. Further, the manual application of the Torque Angle method does not provide an adequate means to check the torque applied across the fasteners. Because of these disadvantages caused by the manual nature of the Torque Angle Method, the more limited deviation of +/-10% cannot readily be recognized and the additional time required of a user to scribe the fastener may cause him to abandon the Torque Angle Method for the Torque Only Method. Permanent angle markings located on a circular dial have been used on an internal hardness tester disclosed in Fietz (3,283,567) but this device is limited to substance hardness determinations. Therefore, a more efficient fastener is necessary to reduce the disadvantages of the Torque Angle Method so that users can more easily utilize the method and receive its advantages over other methods.